Although the clinical effect of polyunsaturated fat on lowering serum lipids is well known, the mechanism of action involved remains unknown. In order to define this aspect of lipid metabolism, two species of monkeys (the atherosclerosis susceptible squirrel monkey and the resistant cebus monkey) will be fed various types of fat and carbohydrate from infancy to provide information on the longitudinal pattern of lipid metabolism and the extent and development of atherosclerosis in these two species. Serum lipids including cholesterol, triglycerides, and free fatty acids, and the lipoprotein metabolism will be studied from birth. Cholesterol turnover and lipoprotein metabolism will be studied with radioisotopes and lymphatic cannulation. Synthesis of these components will be accomplished with gut and liver tissue slices and perfusion techniques. Selected arteries will be assessed for atherosclerosis with electron microscopic techniques. This coordinated approach will supply needed information on lipid metabolism and atherogenesis in nonhuman primates.